THIS invention relates to a process for the selection of HIV-1 subtype (clade) C isolate regulatory/accessory genes, selected HIV-1 subtype C isolate regulatory/accessory genes and modifications and derivatives thereof for use in prophylactic and therapeutic vaccines to produce proteins and polypeptides for the purpose of eliciting protection against HIV infection or disease.
The disease acquired immunodeficiency syndrome (AIDS) is caused by human immunodeficiency virus (HIV). Over 34 million people worldwide are thought to be living with HIV/AIDS, with over 95% of infected people living in developing countries (UNAIDS, 1999). It is estimated that 24.5 million infected people reside in sub-Saharan Africa and that South Africa currently has one of the world's fastest growing HIV-1 epidemics. At the end of 2000, over 24% of pregnant women attending government antenatal clinics in South Africa were HIV positive (Department of Health, 2001). A preventative vaccine is considered to be the only feasible way to control this epidemic in the long term.
HIV shows remarkable genetic diversity that has confounded the development of a vaccine. The molecular basis of variation resides in the viral enzyme reverse transcriptase which not only introduces an error every round of replication, but also promotes recombination between viral RNAs. Based on phylogenetic analysis of sequences, HIV has been classified into a number of groups: the M (major group) which comprises subtypes A to H and K, the O (outlier group) and the N (non-M, non-O group). Recently recombinant viruses have been more frequently identified and there are a number which have spread significantly and established epidemics (circulating recombinant forms or CRF) such as subtype A/G recombinant in West Africa, and CRF A/E recombinant in Thailand (Robertson et al., 2000).
Subtype C predominates in the Southern African region which includes Botswana, Zimbabwe, Zambia, Malawi, Mozambique and South Africa. In addition, increasing numbers of subtype C infections are being detected in the Southern region of Tanzania. This subtype also predominates in Ethiopia and India and is becoming more important in China.
A possible further obstacle to vaccine development is that the biological properties of HIV change as disease progresses. HIV requires two receptors to infect cells, the CD4 and co-receptors of which CCR5 and CXCR4 are the major co-receptors used by HIV-1 strains. The most commonly transmitted phenotype is non-syncytium inducing (NSI), macrophage-tropic viruses that utilise the CCR5 co-receptor for entry (R5 viruses). Langerhans cells in the mucosa are thought to selectively pick up R5 variants at the portal of entry and transport them to the lymph nodes where they undergo replication and expansion. As the infection progresses, viruses evolve that have increased replicative capacity and the ability to grow in T cell lines. These syncytium-inducing (SI) T-tropic viruses use CXCR4 in conjunction with or in preference to CCR5, and in some cases also use other minor co-receptors (Connor et al., 1997, Richman & Bozzette, 1994). However HIV-1 subtype C viruses appear to be unusual in that they do not readily undergo this phenotypic switch, as R5 viruses are also predominant in patients with advanced AIDS (Bjorndal et al., 1999, Peeters et al., 1999, Tscherning et al., 1998, Scarlatti et al., 1997).
An HIV vaccine aims to elicit both a CD8+ cytotoxic T lymphocyte (CTL) immune response as well as a neutralizing antibody response. Many current vaccine approaches have primarily focused on inducing a CTL response. It is thought that the CTL response may be more important as it is associated with the initial control of viral replication after infection, as well as control of replication during disease, and is inversely correlated with disease progression (Koup et al., 1994, Ogg et al., 1999 Schmitz et al., 1999). The importance of CTL in protecting individuals from infection is demonstrated by their presence in highly exposed seronegative individuals, for example certain sex-workers in Kenya (Rowland-Jones et al., 1998).
Knowledge of genetic diversity is highly relevant to the design of vaccines aiming at eliciting a cytotoxic T-lymphocyte (CTL) response. There are many CTL epitopes in common between viruses (HIV Molecular Immunology Database, 1998). In addition, several studies have now shown that there is a cross-reactive CTL response: individuals vaccinated with a subtype B-based vaccine could lyse autologous targets infected with a diverse group of isolates (Ferrari et al., 1997); and CTLs from non-B infected individuals could lyse subtype B-primed targets (Betts et al. 1997; Durali et al, 1998). A comparison of CTL epitopes in the HIV-1 sequence database indicated that there is a greater conservation of cytotoxic T epitopes within a subtype compared to between subtypes and that there will be a greater chance of a CTL response if the challenge virus is the same subtype as the vaccine strain.
It is thought that the regulatory genes of HIV are extremely important in eliciting an immune response. Tat, Rev and Nef are all expressed early in the infectious cycle and would thus provide targets for cytotoxic T lymphocytes (CTLs) early in infection, possibly allowing virus infected cells to be destroyed before the virus can spread (Klotman et. al., 1991; Addo et. al., 2001). In addition, there is promising data showing that the Tat protein elicits an effective immune response in HIV-infected, asymptomatic people (Calarota et. al., 1999; Calarota et. al., 2001). It has recently been reported that the Tat protein is one of the first to undergo escape from CTL in infected macaques (Allen et. al., 2000). This indicates that there is immune pressure on tat and that there is an early response to the Tat protein.
Viral strains used in the design of a vaccine need to be shown by genotypic analysis to be representative of the circulating strains and not an unusual or outlier strain. In addition, it is important that a vaccine strain also has the phenotype of a recently transmitted virus, which is NSI and uses the CCR5 co-receptor.